Chapter 17: Blood – Structure, Function, and Clinical Relevance

Blood: Composition and Physical Characteristics

Overview

Blood is a specialized connective tissue with unique physical and chemical properties. It plays a vital role in maintaining homeostasis and transporting substances throughout the body.

• Composition: 55% plasma, <1% leukocytes (white blood cells), 45% erythrocytes (red blood cells). The percentage of erythrocytes is known as the hematocrit.

• pH: 7.35 - 7.45 (slightly alkaline).

• Volume: Healthy males: 5-6 liters; Healthy females: 4-5 liters.

• Connective Tissue Classification: Blood contains cells, ground substance (plasma), and fibers (coagulation fibers, which are soluble until vessel damage occurs).

Functions of Blood

Distribution, Regulation, and Protection

Blood serves three primary functions: distribution, regulation, and protection.

• Distribution:

  • Delivers oxygen and nutrients to body cells.

  • Transports metabolic waste to lungs and kidneys for elimination.

  • Transports hormones from endocrine organs to target tissues.

• Regulation:

  • Maintains body temperature by absorbing and distributing heat.

  • Maintains normal pH using buffer systems.

  • Maintains adequate fluid volume in the circulatory system.

• Protection:

  • Prevents blood loss via coagulation pathways.

  • Defends against infection through leukocytes, antibodies, and other substances.

Plasma and Serum

Composition and Functions

Plasma is the liquid component of blood, while serum is plasma without clotting proteins.

• Plasma: Straw-colored, sticky fluid; mostly water with dissolved solutes.

• Solutes:

  • Proteins:

    • Albumin: Carrier for molecules, blood buffer, major contributor to osmotic pressure.

    • Globulins: Transport proteins (alpha, beta); antibodies (gamma).

    • Fibrinogen: Precursor to fibrin, the clotting protein.

  • Non-proteins: Nutrients (glucose, amino acids, fatty acids, triglycerides, cholesterol, vitamins), electrolytes (sodium, bicarbonate), respiratory gases (oxygen, carbon dioxide).

• Serum: Plasma with clotting proteins removed.

Erythrocytes (Red Blood Cells)

Structure, Function, and Production

Erythrocytes are specialized for oxygen transport and are produced in the bone marrow.

• Structure: Biconcave disks, anucleate, filled with hemoglobin.

• Function: Transport oxygen (via hemoglobin) and some carbon dioxide.

• Production (Erythropoiesis):

  • Occurs in bone marrow from hemocytoblasts.

  • Stimulated by erythropoietin (from kidneys in response to hypoxia).

  • Requires iron (stored as ferritin, transported by transferrin), vitamin B12, folic acid, and intrinsic factor for B12 absorption.

• Lifespan: ~120 days.

Hemoglobin

Structure and Function

Hemoglobin is the oxygen-carrying protein in erythrocytes.

• Structure: Four polypeptide chains (globin), each with a heme group containing an iron atom.

• Function: Oxygen binds to iron in heme (forms oxyhemoglobin); carbon dioxide binds to globin (forms carbaminohemoglobin).

• Bicarbonate Buffer System: Most CO2 is transported as bicarbonate in plasma.

Leukocytes (White Blood Cells)

Structure, Function, and Production

Leukocytes are defensive cells that protect the body from infection and inflammation.

• Structure: Diverse cell types; capable of diapedesis (movement through capillary walls).

• Function: Defend against pathogens; proliferate during infection.

• Production (Leukopoiesis):

  • Occurs in bone marrow; stimulated by colony stimulating factors (CSFs) from macrophages and T lymphocytes.

  • Hemocytoblasts commit to myeloid (granulocytes, monocytes) or lymphoid (lymphocytes) lineages.

• Clinical Note: WBC count >11,000 cells/ml indicates infection.

Platelets (Thrombocytes)

Structure and Function

Platelets are cell fragments essential for blood clotting.

• Structure: Cytoplasmic fragments from megakaryocytes.

• Function: Stick to damaged vessels, form plugs, and secrete substances for clotting and repair.

Disorders of Blood

Erythrocyte Disorders

• Anemias: Low oxygen-carrying capacity.

  • Insufficient RBCs: Hemorrhagic (blood loss), hemolytic (destruction), aplastic (bone marrow failure).

  • Low Hemoglobin: Iron deficiency (microcytic, pale RBCs), pernicious (B12/intrinsic factor deficiency).

  • Abnormal Hemoglobin: Thalassemias (globin synthesis defect), sickle-cell anemia (abnormal hemoglobin causes sickling).

• Polycythemia: Excess RBCs, increased blood viscosity.

Leukocyte Disorders

• Leukopenia: Low WBC count.

• Leukemia: Cancerous proliferation of dysfunctional WBCs.

  • Acute: From immature cells.

  • Chronic: From more mature cells.

Hemostasis

Stages of Blood Clotting

Hemostasis is the process of stopping bleeding, involving three main stages.

1. Vascular Spasms: Smooth muscle contraction reduces blood loss; triggered by injury, pain, chemicals.

2. Platelet Plug Formation: Platelets adhere to exposed collagen, release granules, attract more platelets, and form a plug.

3. Coagulation: Cascade of reactions leading to fibrin formation. Two pathways (intrinsic and extrinsic) converge to activate thrombin, which converts fibrinogen to fibrin. Calcium is essential for several steps. Clot retraction and repair follow, and clot removal occurs via plasmin activation.

Coagulation Pathways

• Intrinsic Pathway: Triggered by vessel endothelium damage.

• Extrinsic Pathway: Triggered by tissue damage; tissue thromboplastin released.

• Common Pathway: Both pathways converge to activate thrombin.

Key Equation:

$\text{Fibrinogen (soluble)} \xrightarrow{\text{Thrombin}} \text{Fibrin (insoluble)}$

Blood Tests

Diagnostic Tools

• Hematocrit: Percentage of erythrocytes (males: 47%, females: 42% ± 5%).

• Differential WBC Count: Proportion of individual leukocyte types.

• Complete Blood Count (CBC): Counts of all blood cell types, hematocrit, hemoglobin, RBC size.

• Prothrombin Time: Measures blood clotting ability.

ABO and Rh Blood Grouping Systems

Blood Typing and Transfusion Compatibility

Blood groups are determined by antigens (glycoproteins) on RBC membranes. The immune system recognizes foreign antigens, leading to agglutination if incompatible blood is transfused.

• ABO System:

  • Type A: A antigen present.

  • Type B: B antigen present.

  • Type AB: Both A and B antigens present.

  • Type O: Neither A nor B antigens present.

  • Individuals have antibodies against antigens they do not possess.

• Rh System:

  • Rh+ : Rh antigen present.

  • Rh- : Rh antigen absent.

  • Antibodies to Rh antigen form only after exposure.

• Universal Donor: Type O- (no A, B, or Rh antigens).

• Universal Recipient: Type AB+ (no antibodies to A, B, or Rh antigens).

Blood Group Compatibility Table

Blood Type	Antigens Present	Antibodies Present	Can Receive From	Can Donate To
A+	A, Rh	Anti-B	A+, A-, O+, O-	A+, AB+
B+	B, Rh	Anti-A	B+, B-, O+, O-	B+, AB+
AB+	A, B, Rh	None	All types	AB+
O-	None	Anti-A, Anti-B, Anti-Rh	O-	All types
O+	Rh	Anti-A, Anti-B	O+, O-	O+, A+, B+, AB+
AB-	A, B	Anti-Rh	AB-, A-, B-, O-	AB-, AB+
A-	A	Anti-B, Anti-Rh	A-, O-	A-, A+, AB-, AB+
B-	B	Anti-A, Anti-Rh	B-, O-	B-, B+, AB-, AB+

Additional info: Table entries inferred for completeness and clarity.